Cryogenic cooling systems are employed in various demanding applications including military and civilian active and remote sensing, superconducting, and general electronics cooling. Such applications often demand efficient, reliable, and cost-effective cooling systems that can achieve extremely cold temperatures below 80 degrees Kelvin.
Efficient cryogenic cooling systems are particularly important in sensing applications involving high-sensitivity infrared focal plane arrays of electromagnetic energy detectors (FPA's). Generally, an FPA may detect electromagnetic energy radiated or reflected from a scene and convert the detected electromagnetic energy into electrical signals corresponding to an image of the scene. To optimize FPA imaging performance, any FPA detector nonuniformities, such as differences in individual detector offsets, gains, or frequency responses, are corrected. Any spatial or temporal variations in temperature across the FPA may cause prohibitive FPA nonuniformities.
FPA's are often employed in avionics applications, particularly missile targeting applications, where weight, size, and spatial and temporal uniformity of cryogenic cooling systems are important design considerations. An FPA should operate at stable cryogenic temperatures for maximum performance and sensitivity.
Conventionally, a cooling fluid was applied to the FPA via a cooling interface. Heat was transferred to the cooling fluid from the FPA. The heated fluid was then expelled from the missile or re-cooled via a heat exchanger integrated into the FPA. The cooling fluid required a heavy and bulky FPA cooling interface and heat exchanger, which were attached to the FPA mounting assembly. Consequently, the FPA assembly required additional mechanical support to secure the interface, heat exchanger, and cooling fluid. The bulky components and additional support hardware oftentimes required additional cooling, which increased demands placed on the cooling system. The bulky support structure, conventionally thought to improve temperature stability, actually reduced system cooling efficiency. Furthermore, the additional bulky mechanical FPA support hardware caused alignment problems with the on board optical or infrared system during installation and operation, thereby increasing installation and operating costs.
Alternatively, Joule-Thompson cryocoolers (or cryostats) have been employed. A Joule-Thomson cryocooler typically applies a regulated flow of cold gas over the infrared FPA. More specifically, Joule-Thomson cooling occurs when a non-ideal gas expands from high to low pressure at constant enthalpy. The effect can be amplified by using the cooled gas to pre-cool the incoming gas in a heat exchanger. Conventionally, Joule-Thomson heat exchangers have been finned-tube devices or devices made from etched glass such as those manufactured by MMR Technologies, Inc. Disadvantageously, finned tube heat exchangers have a limited heat exchange area and are consequently relatively large and heavy. In addition, glass slide heat exchangers are limited in size and gas flow, which limits the available cooling power. Such conventional methods also incur problems in cost of manufacture.
Undesirably, conventional Joule-Thompson coolers also suffer from relatively short run times because of the size, weight and power penalty associated with a running operation. By increasing the size and weight of the cooler, the additional weight increases the overall operating costs and reduces maneuvering capability and range of the accompanying system. Furthermore, in missile applications, excessive shock or vibration from missile maneuvering may interrupt gas flow, thereby creating potentially prohibitive temperature instabilities, resulting in reduced missile performance.